1. Technical Field
This invention relates generally to rechargeable batteries, and more particularly to smart battery systems with power allocation capabilities.
2. Background Art
Portable electronic devices are becoming as standard an accessory as a wallet or purse. People are carrying cellular phones, personal digital assistants (PDAs), pagers and the like in record numbers. For example, according to the Cellular Telecommunications Industry Association (CTIA), cellular telephone usage in the United States increased 27% between 1999 and 2000. As of December, 2000, there were over 109 million cellular subscribers in the United States alone.
Cellular telephones require batteries for portability. As people talk more and more on cellular telephones, they require more and more energy from batteries. To compound this battery capacity issue, there is a trend in the electronic accessories business to converge devices. For instance, cellular telephones are converging with personal data assistants (PDAs), MPEG-1 Audio Layer 3 (MP3) players, and the like. By way of example, the StarTac(trademark) series phone manufactured by Motorola can be purchased with a clip-on personal data assistant, which is capable of storing appointments, phone lists, and to-do reminders. Additionally, the Visor(trademark) personal data assistant manufactured by Handspring has an expansion slot that allows it to become a cellular telephone. Many cellular telephones have clip-on MP3 players as well.
All of these peripheral devices rely on the phone""s battery for power. With increasing frequency, manufacturers of portable electronic devices are turning to lithium-ion (Li-ion) and lithium-polymer (Li-polymer) rechargeable batteries as the primary power source. These chemistries are advantageous in that they have a high energy density, which means that they offer large amounts of power per unit volume.
The various devices and high-speed circuitry associated with convergent products often consume large amounts of power when operating. This has created an additional strain on an already overtaxed battery. Nothing is more frustrating than missing a casting call for a Broadway musical because your phone battery is dead. When your agent calls for that Broadway audition, you may be very angry with yourself for having listened to the long version of xe2x80x9cStairway to Heavenxe2x80x9d on your clip-on MP3 player instead of having saved the last 10% of your battery for that one additional call.
In an attempt to solve this problem, manufacturers have turned to voltage-sensing fuel gauges to try and inform the user as to how much battery power remains. A problem with lithium-based batteries is that it is hard to measure just how much power is left in them by sensing the voltage. When a primary battery discharges, like an alkaline battery for instance, the voltage drops proportionally with capacity. Thus if the working voltage has dropped by X %, the capacity has dropped in a corresponding amount. To find out how much capacity you have left, all you need to do is measure the voltage and compare it to the initial value.
Lithium-based batteries, by contrast, have a relatively flat discharge characteristic. The voltage stays essentially constant until the battery is xe2x80x9cdeadxe2x80x9d, when the voltage drops very quickly. For this reason, lithium based systems comprising fuel-gauges that measure voltage can have inaccuracies of 25% or more.
A more sophisticated, accurate method involves the use of a xe2x80x9cCoulomb counterxe2x80x9d fuel gauge. This fuel gauge measures the amount of power that has discharged from the battery by integrating current across time (Coulombs of charge). This technique can yield accuracy""s of 1% or better in certain applications. The problem with Coulomb counting fuel gauges is that small portable products, e.g. phones, PDAs, and the like, do not have computing power available to perform power management functions in addition to measuring battery capacity.
There is thus a need for an improved means of managing power in portable electronic devices.